Support structure for free-standing MEMS device and methods for forming the same

ABSTRACT

A microelectromechanical (MEMS) device includes a functional layer including a first material and a deformable layer including a second material. The second material is different from the first material. The deformable layer is mechanically coupled to the functional layer at a junction. The functional layer and the deformable layer have substantially equal internal stresses at the junction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention relates to microelectromechanical systems(MEMS).

2. Description of the Related Art

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and/or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY OF THE INVENTION

In certain embodiments, a microelectromechanical (MEMS) device comprisesa functional layer and a deformable layer. The functional layercomprises a first material. The deformable layer comprises a secondmaterial. The second material is different from the first material. Thedeformable layer is mechanically coupled to the functional layer at ajunction. The functional layer and the deformable layer havesubstantially equal internal stresses at the junction.

In certain embodiments, a microelectromechanical (MEMS) device comprisesmeans for controlling a signal and means for supporting the controllingmeans. The controlling means comprises a first material. The supportingmeans comprises a second material. The second material is different fromthe first material. The supporting means is mechanically coupled to thecontrolling means at a junction. The controlling means and thesupporting means have substantially equal internal stresses at thejunction.

In certain embodiments, a method of manufacturing amicroelectromechanical (MEMS) device on a substrate comprises forming afunctional layer, forming a sacrificial layer over the functional layer,forming a hole in the sacrificial layer to expose a portion of thefunctional layer, and forming a deformable layer over the sacrificiallayer. The functional layer comprises a first material. The sacrificiallayer comprises sacrificial material. The deformable layer comprises asecond material. The second material is different from the firstmaterial. The deformable layer is fused to the functional layer at ajunction in the exposed portion of the functional layer. The functionallayer and the deformable layer have substantially equal internalstresses at the junction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIG. 5A illustrates one exemplary frame of display data in the 3×3interferometric modulator display of FIG. 2.

FIG. 5B illustrates one exemplary timing diagram for row and columnsignals that may be used to write the frame of FIG. 5A.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a cross section of an embodiment of an interferometricmodulator with curved and/or tilted reflective layers.

FIGS. 9A-9F depict cross-sectional views of one method of manufacturingan interferometric modulator with a reflective layer that is not curvedand/or tilted.

FIGS. 10A-10C depict cross-sectional views of another method ofmanufacturing an interferometric modulator with a reflective layer thatis not curved and/or tilted.

FIGS. 11A-11C depict cross-sectional views of yet another method ofmanufacturing an interferometric modulator with a reflective layer thatis not curved and/or tilted.

FIG. 12 is a cross-section of an embodiment of an interferometricmodulator with a reflective layer that is not curved and/or tilted.

FIG. 13 is a cross-section of another embodiment of an interferometricmodulator with a reflective layer that is not curved and/or tilted.

FIG. 14 is a cross-section of yet another embodiment of aninterferometric modulator with a reflective layer that is not curvedand/or tilted.

FIG. 15 is a cross-section of still another embodiment of aninterferometric modulator with a reflective layer that is not curvedand/or tilted.

FIGS. 16A-16D depict cross-sectional views of an example switch with acontact layer that is curved and/or tilted.

FIGS. 17A-17D depict cross-sectional views of an embodiment of a switchwith a contact layer that is not curved and/or tilted.

FIGS. 18A-18D depict cross-sectional views of another embodiment of aswitch with a contact layer that is not curved and/or tilted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

In certain embodiments, a junction of a deformable layer comprising afirst material and a functional layer comprising a second material isprovided. The functional layer and the deformable layer havesubstantially equal internal stresses at the junction. The lack ofstress gradients at the junction decreases the curvature and/or tilt ofthe functional layer. Decreasing curvature and/or tilt is desirable, forexample, to provide a substantially flat reflective layer and to makebetter contact or parallel spacing between electrodes. In someembodiments, the functional layer comprises a bilayer of a reflective orconductive material and a deformable material. In some embodiments, thefunctional layer comprises a graded composition (e.g., the firstmaterial having graded internal stresses or an alloy comprising thefirst material and the second material) that varies from a first side ofthe functional layer proximate to the deformable layer to a second sideof the functional layer distal from the deformable layer. In someembodiments, the functional layer comprises a bilayer of a reflective orconductive material and a layer with a graded composition that variesfrom the reflective or conductive layer to the deformable layer.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent, and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the cavity 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. Thus, there exists awindow of applied voltage, about 3 to 7 V in the example illustrated inFIG. 3, within which the device is stable in either the relaxed oractuated state. This is referred to herein as the “hysteresis window” or“stability window.” For a display array having the hysteresischaracteristics of FIG. 3, the row/column actuation protocol can bedesigned such that during row strobing, pixels in the strobed row thatare to be actuated are exposed to a voltage difference of about 10volts, and pixels that are to be relaxed are exposed to a voltagedifference of close to zero volts. After the strobe, the pixels areexposed to a steady state voltage difference of about 5 volts such thatthey remain in whatever state the row strobe put them in. After beingwritten, each pixel sees a potential difference within the “stabilitywindow” of 3-7 volts in this example. This feature makes the pixeldesign illustrated in FIG. 1 stable under the same applied voltageconditions in either an actuated or relaxed pre-existing state. Sinceeach pixel of the interferometric modulator, whether in the actuated orrelaxed state, is essentially a capacitor formed by the fixed and movingreflective layers, this stable state can be held at a voltage within thehysteresis window with almost no power dissipation. Essentially nocurrent flows into the pixel if the applied potential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4embodiment, actuating a pixel involves setting the appropriate column to−V_(bias), and the appropriate row to +ΔV, which may correspond to −5volts and +5 volts, respectively Relaxing the pixel is accomplished bysetting the appropriate column to +V_(bias), and the appropriate row tothe same +ΔV, producing a zero volt potential difference across thepixel. In those rows where the row voltage is held at zero volts, thepixels are stable in whatever state they were originally in, regardlessof whether the column is at +V_(bias), or −V_(bias). As is alsoillustrated in FIG. 4, it will be appreciated that voltages of oppositepolarity than those described above can be used, e.g., actuating a pixelcan involve setting the appropriate column to +V_(bias), and theappropriate row to −ΔV. In this embodiment, releasing the pixel isaccomplished by setting the appropriate column to −V_(bias), and theappropriate row to the same −ΔV, producing a zero volt potentialdifference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 44, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding and vacuum forming. In addition, the housing 41 may be made fromany of a variety of materials, including, but not limited to, plastic,metal, glass, rubber, and ceramic, or a combination thereof. In oneembodiment, the housing 41 includes removable portions (not shown) thatmay be interchanged with other removable portions of different color, orcontaining different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43, which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g., filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment, the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS, or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g., an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a bi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, or a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some embodiments, control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some embodiments, controlprogrammability resides in the array driver 22. Those of skill in theart will recognize that the above-described optimizations may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as support posts. Theembodiment illustrated in FIG. 7D has support post plugs 42 upon whichthe deformable layer 34 rests. The movable reflective layer 14 remainssuspended over the cavity, as in FIGS. 7A-7C, but the deformable layer34 does not form the support posts by filling holes between thedeformable layer 34 and the optical stack 16. Rather, the support postsare formed of a planarization material, which is used to form supportpost plugs 42. The embodiment illustrated in FIG. 7E is based on theembodiment shown in FIG. 7D, but may also be adapted to work with any ofthe embodiments illustrated in FIGS. 7A-7C, as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

Optimization of the structural design and materials used for thereflective layer (or “mirror layer”) 14 and the deformable layer (or“mechanical layer”) 34 may result in different materials being used forthe reflective layer 14 and the deformable layer 34. Different materialsmay have different properties, such as residual stresses, which cancause curvature and/or tilt in the reflective layer 14. For example,crystalline nickel has an intrinsic crystal lattice stress of about 350megapascals (MPa) and crystalline aluminum has an intrinsic crystallattice stress of about 50 MPa. Because the residual stresses aredifferent, an interface between nickel and aluminum will have a stressgradient, which will exert tensile or compressive forces, therebycausing curvature and/or tilt (or “launching” and “deflection”) of thematerial that is more pliable or compliant (e.g., aluminum as comparedto nickel). As illustrated in FIG. 8, the interface between differentmaterials with mismatched crystal lattices for the reflective layer 14and the deformable layer 34, for example aluminum and nickel,respectively, can cause curvature and/or tilt of the reflective layer14. FIG. 8 generally corresponds to the embodiment depicted by FIG. 7Chaving different materials for the reflective layer 14 and thedeformable layer 34. The use of different materials for the reflectivelayer 14 and the deformable layer 34 may also result in curvature and/ortilt of the reflective layer 14 in the embodiments depicted in FIGS. 7Dand 7E, as well.

Another property that may be different between different materials iscoefficient of thermal expansion. When a device comprising differentmaterials for the reflective layer 14 and the deformable layer 34 isheated or cooled, thermal stresses due to different amounts of thermalexpansion or contraction between the materials used for the reflectivelayer 14 and the deformable layer 34 can contribute to the curvatureand/or tilt of the reflective layer 14. Thus, the magnitude of curvatureand/or tilt is a function of temperature in some embodiments.

In certain embodiments, stress distribution in the reflective layer 14is non-uniform because there is a stress gradient in the portion of thereflective layer 14 forming the interface 36, but there is no stressgradient in the other portions of the reflective layer 14. Thus, asmaller interface 36 between the deformable layer 34 and the reflectivelayer 14 can result in increasingly non-uniform stress distributionacross the reflective layer 14, thereby increasing the curvature and/ortilt of the reflective layer 14.

Curvature and tilt of the reflective layer 14 may affect the size of thehysteresis window and the optical properties of the reflective layer 14.As described above, the row/actuation protocol may be set according to ahysteresis window, so a change in the hysteresis window may cause thedevice to function improperly or to fail.

Even if the device works within a given hysteresis window, the changedoptical properties may adversely affect performance of a displaycomprising the device. Preferably, the surface of the reflective layer14 facing the substrate 20 is substantially parallel to the opticalstack 16. However, curvature and/or tilt of the reflective layer 14 maycause the surface of the reflective layer 14 facing the substrate 20 tobecome non-parallel to the optical stack 16. The reflective layer 14 mayreflect differing amounts of visible light across its area, distortingwhether the reflective layer 14 is in the “on” or “off” position.

In certain embodiments, the interface between the different materialsfor the reflective layer 14 and the deformable layer 34 is positionedaway from the reflective layer 14, thereby decreasing the curvatureand/or tilt of the reflective layer 14. The stress gradients due toresidual stresses, coefficients of thermal expansion, and non-uniformstress distribution can thereby be positioned away from the reflectivelayer 14 or can be substantially eliminated.

FIG. 9F depicts a cross-sectional view of a device in which theinterface 95 between the different materials for the reflective layer 14and the deformable layer 34 is spaced from the reflective layer 14. Thereflective layer 14 comprises a first material. The first material ispreferably optimized with respect to optical properties, such as beingreflective to light. Examples of the first material that are reflectiveto light include, but are not limited to, silver, gold, copper,aluminum, and alloys thereof. The first material may be optimized withrespect to optical properties, such as being reflective to ultraviolet(UV) or infrared (IR) light. Examples of the first material that arereflective to UV and/or IR light include, but are not limited to, zinc,platinum, rhodium, and titanium. The deformable layer 34 comprises asecond material different from the first material. The second materialis preferably optimized with respect to desired mechanical properties,such as being controllably deformable and providing suitable restoringforces. Examples of the second material include, but are not limited to,nickel, aluminum, aluminum alloys, chromium, nickel alloys (e.g., NiV),and iron.

In some embodiments, the deformable layer 34 is supported by supportposts 33. The support posts 33 preferably comprise a rigid material thatdoes not significantly deform. Examples of the rigid material include,but are not limited to, silicon oxide (SiO₂), silicon nitride (SiN_(x)),silicon oxynitride (SiO_(x)N_(y)), aluminum oxide (Al₂O₃), spin-on glass(“SOG”), and spin-on dielectric (“SOD”). In certain embodiments, thesupport posts 33 comprise a conductive material with an insulatingmaterial disposed between the support posts 33 and the deformable layer34. Other configurations, such as those depicted in FIGS. 7D and 7E, maycomprise other support posts 42 that support the deformable layer 34.

Referring again to FIG. 9F, the device further comprises a connectingelement 94 mechanically coupled to the deformable layer 34 and thereflective layer 14. In certain embodiments, the connecting element 94is fused to the reflective layer 14. In certain embodiments, theconnecting element 94 comprises the first material. The connectingelement 94 preferably comprises conductive material, for example, butnot limited to, aluminum, although the connecting element 94 maycomprise insulating or semiconductive material. The connecting element94 may or may not comprise a deformable material. The connecting element94 and the deformable layer 34 form an interface 95 between the firstmaterial and the second material. The interface 95 is spaced from thereflective layer 14.

In certain embodiments, the connecting element 94 does not comprise thefirst material, but comprises a material with a substantially similarintrinsic crystal lattice stress and/or coefficient of thermal expansionto the portion of the reflective element 14 to which the connectingelement is mechanically coupled (e.g., fused). When the residualstresses and coefficient of thermal expansion are substantially similar,the stress gradients are mitigated or eliminated, thereby mitigating oreliminating curvature and/or tilt of the reflective layer 14.

In certain embodiments, neither the reflective layer 14 nor theconnecting element 94 is fused to the deformable layer 34. As such, thereflective layer 14 and the connecting element 94 may be described as“floating.” When a force such as gravity and/or an electrostatic forcedue to an electric field causes the reflective layer 14 and theconnecting element 94 to move towards the substrate 20 (e.g., in thedirection of the distributed electrostatic force due to an electricfield schematically indicated by arrows 96), the portions 93 of theconnecting element 94 that overlap the cantilevers 37 of the deformablelayer 34 impede the reflective layer 14 from falling onto the opticalstack 16. Upon activation, during which the cantilevers 37 of thedeformable layer 34 are deflected, the reflective layer 14 is movedcloser to the optical stack 16, as depicted on the right side of FIG.9F. Upon relaxation, the electrostatic force due to an electric field isremoved, and the deformable layer 34 returns to the relaxed state andthe reflective layer 14 returns to the relaxed position, as depicted onthe left side of FIG. 9F.

Once the connecting element 94 and the deformable layer 34 are incontact, adhesion forces (e.g., van der Waals forces) between theconnecting element 94 and the deformable layer 34 keep the reflectivelayer 14 from moving translationally and keep the connecting element 94from disconnecting from the deformable layer 34. A larger area of theinterface 95 between the connecting element 94 and the deformable layer34 results in stronger adhesion forces holding the connecting element 94to the deformable layer 34. In some embodiments, the terminal edges ofthe cantilevers 37 of the deformable layer 34 near the interface 95 arecurled downward or upward (e.g., as illustrated in FIG. 9F) due to themismatched residual stresses between the materials of the connectingelement 94 and the deformable layer 34. For example, in an embodiment inwhich the deformable layer 34 comprises nickel with tensile stress andthe posts 33 comprise silicon dioxide with compressive stress, theterminal edges of the cantilevers 37 of the deformable layer 34 arecurled upwards. The area of the interface 95 may depend on the width andprofile of the cantilevers 37 of the deformable layer 34, the width andprofile of the portions 93 of the connecting element 94, the materialsused for the deformable layer 34 and the connecting element 94,photolithography critical dimension and alignment considerations, thesmoothness of the top of the deformable layer 34 and the bottom of theconnecting element 94, and other factors. The interface 95 in certainembodiments has an area between about 80 and 1,440 square microns, andin certain other embodiments has an area between about 144 and 512square microns. Other areas of the interface 95 are also possible.

In certain embodiments, the interface 95 between the connecting element94 and the deformable layer 34 is spaced from the interface between theposts 33 and the deformable layer 34. The rigidity of the posts 33 canimpede the deformable layer 34 from deforming. In certain embodiments,the posts 33 have cantilever portions 35, and the sizes of thesecantilever portions 35 of the posts 33 are minimized in order toincrease the area available for the interface 95.

FIGS. 9A through 9F illustrate cross-sectional views of one method ofmanufacturing an interferometric modulator in accordance with certainembodiments described herein. It will be appreciated that someembodiments may add, delete, rearrange, change, substitute, or otherwisemodify the processes described herein.

FIG. 9A depicts an interferometric modulator after the formation on asubstrate 20 of an optical stack 16, a reflective layer 14 comprising afirst material, a sacrificial layer 17 over the reflective layer 14, anda deformable layer 34 comprising a second material over the sacrificiallayer 17. Removal of the sacrificial layer 17 at this point would resultin the structure illustrated in FIG. 8. Because the reflective layer 14and the deformable layer 34 may comprise different materials, thisdevice may experience curvature and/or tilt of the reflective layer 14,as described above.

Alternative processing can result in interferometric modulators withreflective layers 14 that do not substantially experience curvatureand/or tilt resulting from the use of different materials for thedeformable layer 34 and the reflective layer 14. For example, in certainembodiments, a connecting element comprising the first material that ismechanically coupled (e.g., fused) to the reflective layer 14 is formed,and is mechanically coupled to the deformable layer 34 to form aninterface between the first material and the second material that isspaced from the reflective layer 14.

FIG. 9B depicts the device of FIG. 9A after a hole 90 has been formedthrough the deformable layer 34 to uncover at least a portion of thereflective layer 14. Formation of the hole 90 also results in formationof the cantilevers 37 of the deformable layer 34 that at least partiallyextend beyond the cantilever portions 35 of the posts 33. In certainembodiments, the deformable layer 34 contacts the reflective layer 14(as depicted by FIG. 9A) due to the formation of a hole through thesacrificial layer 17. The hole through the sacrificial layer 17 may beformed, for example, at the same time that the sacrificial layer 17 ispatterned to define individual devices. The hole 90 may be formed bycreating a pattern on top of the deformable layer 34 (e.g., withphotoresist) and removing the portions of the deformable layer 34 notcovered by the patterned material (e.g., by wet and/or dry etching). Inembodiments in which the hole 90 has tapered sides through thedeformable layer 34, removal of the deformable layer 34 is preferablyperformed by wet etching and/or anisotropic dry etching. In someembodiments, some of the sacrificial layer 17 is also removed.

In certain embodiments, no hole is formed in the sacrificial material 17prior to deposition of the deformable layer 34. As such, the deformablelayer 34 does not contact the reflective layer 14 (e.g., due to thesacrificial layer 17 remaining between the deformable layer 34 and thereflective layer 14). The hole 90 may be formed through both thedeformable layer 34 and the sacrificial layer 17 by creating a patternon top of the deformable layer 34 (e.g., with photoresist), removing theportions of the deformable layer 34 not covered by the patternedmaterial (e.g., by wet and/or dry etching), and removing the portions ofthe sacrificial layer 17 under the removed portions of the mechanicallayer 34 in the same or a subsequent removal process. In embodiments inwhich the hole 90 has tapered sides through the deformable layer 34 andthe sacrificial layer 17, removal of the deformable layer 34 and/or thesacrificial layer 17 is preferably performed by wet etching and/oranisotropic dry etching.

FIG. 9C depicts the device of FIG. 9B after a sacrificial spacer 92 hasbeen formed in the hole 90. Formation of the sacrificial spacer 92leaves at least a portion of the reflective layer 14 uncovered such thatmaterials deposited over the sacrificial spacer 92 may be mechanicallycoupled (e.g., fused) to the reflective layer 14. In certainembodiments, the sacrificial spacer 92 overlaps the cantilevers 37 ofthe deformable layer 34 but does not overlap the cantilever portions 35of the posts 33. In certain embodiments, the sacrificial spacer 92 isformed by depositing the sacrificial spacer material (e.g., by chemicalvapor deposition, physical vapor deposition, atomic layer deposition,etc.), creating a pattern on top of the sacrificial spacer material(e.g., with photoresist), and removing the portions of the sacrificialspacer not covered by the patterned material (e.g., by wet and/or dryetching).

FIG. 9D depicts the device of FIG. 9C after a connecting element 94 hasbeen formed. The connecting element 94 is mechanically coupled (e.g.,fused) to the reflective layer 14 at the portion of the reflective layer14 not covered by the sacrificial spacer 92. The connecting element 94includes portions 93 that at least partially overlap the cantilevers 37of the deformable layer 34 but do not overlap the cantilever portions 35of the posts 33. In certain embodiments, the connecting element 94 isformed by depositing the first material at least partially within thesacrificial spacer 92 and mechanically coupled (e.g., fused) to thereflective layer 14 (e.g., by chemical vapor deposition, physical vapordeposition, atomic layer deposition, etc.), creating a pattern on top ofthe first material (e.g., with photoresist), and removing the portionsof the first material not covered by the patterned material (e.g., bywet and/or dry etching).

FIG. 9E depicts the device of FIG. 9D after the sacrificial layer 17 andthe sacrificial spacer 92 have been removed. The connecting element 94remains mechanically coupled (e.g., fused) to the reflective layer 14,but the connecting element 94 is not mechanically coupled to thedeformable layer 34. In this state, the reflective layer 14 can be saidto be “floating.” In certain embodiments, the sacrificial layer 17and/or the sacrificial spacer 92 are removed by etching (e.g., by wetand/or dry etching), either in the same process or in differentprocesses. In some embodiments, the sacrificial layer 17 and thesacrificial spacer 92 are removed by etching with xenon difluoride(XeF₂). After removal of the sacrificial layer 17 and the sacrificialspacer 92, the stresses between the posts 33 and the deformable layer 34can cause the more compliant material, typically the deformable layer34, to experience launching and/or deflection. In certain embodiments,the launching and/or deflection can cause the edges of the cantilevers37 curl upwards. Such curling of the edges of the cantilevers 37 can beadvantageous because the cantilevers 37 may thereby make contact withthe connecting element 93 when it is floating.

FIG. 9F depicts the device of FIG. 9E after the connecting element 94has moved towards the substrate 20, for example due to gravity and/or anelectrostatic force due to an electric field, thereby forming aninterface 95 between the first material and the second material. Whenthe connecting element 94 contacts the deformable layer 34, theconnecting element 94 may adhere to the deformable layer 34 as describedabove. The resulting cavity 19 between the reflective layer 14 and theoptical stack 16 provides space in which the reflective layer 14 canmove between the actuated position and the relaxed position in responseto the application of a voltage, as described above.

FIG. 10C depicts a cross-sectional view of another device in which aninterface 105 between the different materials for the reflective layer14 and the deformable layer 34 is spaced from the reflective layer 14.Unlike the embodiment depicted in FIG. 9F, the connecting element 104 isfused to the deformable layer 34 at the interface 105, so the area ofthe interface 105 is not optimized to maximize adhesion. The interface105 in certain embodiments has an area between about 60 and 780 squaremicrons, and in certain other embodiments has an area between about 80and 275 square microns. Other areas of the interface 105 are alsopossible. Similar to the embodiment depicted in FIG. 9F, the interface105 between the first material and the second material is spaced fromthe reflective layer 14.

When the reflective layer 14 is in a relaxed position, as depicted onthe left side of FIG. 10C, the deformable layer 34 is in a relaxedstate. When the reflective layer 14 is in an actuated position, asdepicted on the right side of FIG. 10C, the deformable layer 34 is in adeformed state. The material that the deformable layer 34 comprises canbe optimized to restorably deform when a force (e.g., in the directionof a distributed electrostatic force due to an electric field asschematically indicated by arrows 106) attracts the reflective layer 14towards the substrate 20. When the force is removed, the deformablelayer 34 returns to the relaxed state and the reflective layer 14returns to the relaxed position.

FIGS. 10A through 10C illustrate cross-sectional views of another methodof manufacturing an interferometric modulator in accordance with certainembodiments described herein. FIG. 10A depicts the device of FIG. 9Bafter a sacrificial spacer 102 has been formed in the hole 90. Thesacrificial spacer 102 leaves at least a portion of the reflective layer14 uncovered such that materials deposited over the sacrificial spacer102 may be mechanically coupled (e.g., fused) with the reflective layer14. In certain embodiments, the sacrificial spacer 102 overlaps thecantilevers 37 of the deformable layer 34 but does not overlap thecantilever portions 35 of the posts 33. In certain embodiments, thesacrificial spacer 102 is formed by depositing the sacrificial spacermaterial (e.g., by chemical vapor deposition, physical vapor deposition,atomic layer deposition, etc.), creating a pattern on top of thesacrificial spacer material (e.g., with photoresist), and removing theportions of the sacrificial spacer material not covered by the patternedmaterial (e.g., by wet and/or dry etching).

FIG. 10B depicts the device of FIG. 10A after a connecting element 104has been formed. The connecting element 104 is mechanically coupled(e.g., fused) to the reflective layer 14 at the portion of thereflective layer 14 not covered by the sacrificial spacer 102. Theconnecting element 104 overlaps the sacrificial spacer 102 such that theconnecting element 104 is fused to the deformable layer 34 at theinterface 105 between the first material and the second material. Theconnecting element 104 includes portions 103 that at least partiallyoverlap the cantilevers 37 of the deformable layer 34 but do not overlapthe cantilever portions 35 of the posts 33. In certain embodiments, theconnecting element 104 is formed by depositing the first material atleast partially within the sacrificial spacer 102 and mechanicallycoupled (e.g., fused) to the reflective layer 14 (e.g., by chemicalvapor deposition, physical vapor deposition, atomic layer deposition,etc.), creating a pattern on top of the first material (e.g., withphotoresist), and removing the portions of the first material notcovered by the patterned material (e.g., by wet and/or dry etching).

FIG. 10C depicts the device of FIG. 10B after the sacrificial layer 17and the sacrificial spacer 102 have been removed. The connecting element104 remains mechanically coupled (e.g., fused) to the reflective layer14 and fused to the deformable layer 34. In certain embodiments, thesacrificial layer 17 and/or the sacrificial spacer 102 are removed byetching (e.g., by wet etching), either in the same process or indifferent processes. The resulting cavity 19 between the reflectivelayer 14 and the optical stack 16 provides space in which the reflectivelayer 14 can move between the actuated position and the relaxed positionin response to the application of a voltage, as described above.

FIG. 11C depicts a cross-sectional view of another device in which aninterface 115 between the different materials for the reflective layer14 and the deformable layer 34 is spaced from the reflective layer 14.Unlike the embodiments depicted in FIGS. 9F and 10C, the interface 115between the connecting element 114 and the deformable layer 34 is belowthe deformable layer 34. Similar to the embodiment depicted in FIG. 10C,the connecting element 114 is fused to the deformable layer 34, so thearea of the interface 115 is not optimized to maximize adhesion. Similarto the embodiment depicted in FIGS. 9F and 10C, the interface 115between the first material and the second material is spaced from thereflective layer 14.

When the reflective layer 14 is in a relaxed position, as depicted onthe left side of FIG. 11C, the deformable layer 34 is in a relaxedstate. When the reflective layer 14 is in an actuated position, asdepicted on the right side of FIG. 11C, the deformable layer 34 is in adeformed state. The second material 112 that the deformable layer 34comprises can be optimized to restorably deform when a force (e.g., inthe direction of a distributed electrostatic force due to an electricfield as schematically indicated by arrows 116) attracts the reflectivelayer 14 towards the substrate 20. When the force is removed from thereflective layer 14, the deformable layer 34 returns to the relaxedstate and the reflective layer 14 returns to the relaxed position.

FIGS. 11A through 11C illustrate cross-sectional views of yet anothermethod of manufacturing an interferometric modulator in accordance withcertain embodiments described herein. FIG. 11A depicts an embodiment inwhich forming the deformable layer 34 and the connecting element 114comprises depositing a first material 111 so that a portion of the firstmaterial 111 is mechanically coupled (e.g., fused) to the reflectivelayer 14 and depositing a second material 112 on top of the firstmaterial 111. In certain embodiments, the first material 111 comprisesthe same material as the first material of the reflective layer 14 andthe second material 112 comprises the same material as the secondmaterial of the deformable layer 34. The first material 111 ismechanically coupled (e.g., fused) to the reflective layer 14. Incertain embodiments, the first material 111 does not substantiallyoverlap the entire structure of the posts 33 in order to increase therigidity of the posts 33. For example, the first material 111 may bepatterned such that the first material 111 overlaps the cantileverportions 35 of the posts 33 but not the other portions of the posts 33.

FIG. 11B depicts the device of FIG. 11A after a connecting element 114has been formed. The connecting element 114 in the embodiment depictedby FIG. 11B is further formed by removing at least a portion of thesecond material 112 formed over the portion of the first material 111that is mechanically coupled to the reflective layer 14. The interface115 between the first material 111 and the second material 112 is belowthe deformable layer 34 and is spaced from the reflective layer 14. Incertain embodiments, the connecting element 114 is formed by creating apattern on top of the second material 112 (e.g., with photoresist) andremoving the portions of the second material 112 not covered by thepatterned material (e.g., by wet and/or dry etching). In someembodiments, removing the portions of the second material 112 notcovered by the patterned material comprises selectively etching thesecond material 112 such that at least some of the first material 111beneath the removed portions of the second material 112 remains. In someembodiments, the first material 111 is thick enough that the firstmaterial remains fused to the reflective layer 14 even if some of thefirst material 111 is removed. The remaining second material 112 ispreferably spaced from the area where the first material 111 is fused tothe reflective layer 14, but is also preferably large enough to providesuitable restoring forces.

FIG. 11C depicts the device of FIG. 11B after the sacrificial layer 17has been removed. The connecting element 114 remains mechanicallycoupled (e.g., fused) to the reflective layer 14 and is fused to thesecond material 112. In certain embodiments, the sacrificial layer 17 isremoved by etching (e.g., by wet etching). The resulting cavity 19between the reflective layer 14 and the optical stack 16 provides spacein which the reflective layer 14 to move between the actuated positionand the relaxed position in response to the application of a voltage, asdescribed above.

In certain embodiments, as schematically illustrated by FIG. 12, thethickness of the reflective layer 14 is selected so that the stressgradients due to residual stresses, coefficients of thermal expansion,and non-uniform stress distribution do not cause significant curvatureand/or tilt of the reflective layer 14. In some embodiments, a thicknessof more than about one micron is advantageously used. While FIG. 12depicts a configuration in which the interface 125 between the firstmaterial and the second material is adjacent to the reflective layer 14,in certain embodiments, the thickness of the reflective layer 14 isselected to reduce any curvature and/or tilt of the reflective layer 14in configurations as described above with the interface spaced from thereflective layer 14.

When the reflective layer 14 is in a relaxed position, as depicted onthe left side of FIG. 12, the deformable layer 34 is in a relaxed state.When the reflective layer 14 is in an actuated position, as depicted onthe right side of FIG. 12, the deformable layer 34 is in a deformedstate. The deformable layer 34 deforms when a force (e.g., in thedirection of a distributed electrostatic force due to an electric fieldas schematically indicated by arrows 126) attracts the reflective layer14 towards the substrate 20. When the force is removed from thereflective layer 14, the deformable layer 34 returns to the relaxedstate and the reflective layer 14 returns to the relaxed position.

In certain embodiments, the reflective layer 14 comprises a firstmaterial and the deformable layer 34 comprises a second materialdifferent from the first material. The reflective layer 14 and thedeformable layer 34 form a junction, and the reflective layer 14 and thedeformable layer 34 have substantially equal internal stresses at thejunction. As used herein, the term “substantially equal internalstresses” is to be given its broadest possible meaning, including, butnot limited to, internal stresses that are similar enough that thecurvature and/or tilt of the reflective layer 14 is suitably decreased.The substantial equality of the internal stresses depends on factorssuch as materials, thicknesses, contact area, and coefficients ofthermal expansion. In certain embodiments, the difference in internalstresses between the reflective layer 14 and the deformable layer 34that is substantially equal at the junction is less than about 150 MPa,less than about 60 MPa, and less than about 10. It will be appreciatedthat substantially equal may also mean that the internal stresses arethe same, for example and without limitation when the reflective layer14 and the deformable layer 34 comprise the same material at thejunction. In some embodiments, the reflective layer 14 and thedeformable layer 34 have substantially equal coefficients of thermalexpansion at the junction.

FIG. 13 depicts a cross-sectional view of an embodiment of a device inwhich the reflective layer 14 comprises a first material and in whichthe deformable layer 34 comprises a second material different from thefirst material. The deformable layer 34 is mechanically coupled to thereflective layer 14 at a junction 135. The reflective layer 14 and thedeformable layer 34 have substantially equal internal stresses at thejunction 135. The reflective layer 14 comprises a reflective layer 132on a side of the reflective layer 14 facing away from the deformablelayer 34.

In some embodiments, the reflective layer 14 comprises a bilayercomprising a reflective layer 132 comprising the first material on aside of the reflective layer 14 facing away from the deformable layer 34and a layer 131 of the second material. At the junction 135, thedeformable layer 34 comprises the second material and the reflectivelayer 14 comprises the second material, so the internal stresses of thedeformable layer 34 and the reflective layer 14 are substantially equal.In certain embodiments, the first material comprises aluminum and thesecond material comprises nickel. The term “bilayer” is not to belimiting, and the reflective layer 14 may comprise more than two layers,for example by inserting a third layer between the reflective layer 132and the layer 131 of the second material. Although there may be somestress gradients at an interface between the reflective layer 132 andthe layer 131 of the second material within the reflective layer 14, thereflective layer 132 is thin compared to the layer 131 of the secondmaterial. In some embodiments, the reflective layer 132 comprises lessthan about 20%, less than about 10%, or less than about 3% of thethickness of the reflective layer 14. In certain embodiments, formationof the bilayer comprises deposition of the reflective layer 132 anddeposition of the layer 131 of the second material over the reflectivelayer 132.

When the reflective layer 14 is in a relaxed position, as depicted onthe left side of FIG. 13, the deformable layer 34 is in a relaxed state.When the reflective layer 14 is in an actuated position, as depicted onthe right side of FIG. 13, the deformable layer 34 is in a deformedstate. The second material can be optimized to restorably deform when aforce (e.g., in the direction of a distributed electrostatic force dueto an electric field as schematically indicated by arrows 136) attractsthe reflective layer 14 towards the substrate 20. When the force isremoved from the reflective layer 14, the deformable layer 34 returns tothe relaxed state and the reflective layer 14 returns to the relaxedposition (e.g., as illustrated on the left side of FIG. 13).

FIG. 14 depicts a cross-sectional view of another embodiment of a devicein which the reflective layer 14 comprises a first material and in whichthe deformable layer 34 comprises a second material different from thefirst material. The deformable layer 34 is mechanically coupled to thereflective layer 14 at a junction 145. The reflective layer 14 and thedeformable layer 34 have substantially equal internal stresses at thejunction 145. The reflective layer 14 has a graded composition thatvaries from a first side 141 proximate to the deformable layer 34 to asecond side 142 of the reflective layer 14 distal from the deformablelayer 34. As used herein, the term “graded” is to be given its broadestpossible definition, including, but not limited to, having a compositionthat varies generally continuously (e.g., linearly, non-linearly) acrossits thickness and having a composition that varies in a step-wise manneror non-continuously across its thickness. The reflective layer 14 isreflective at the second side 142 of the reflective layer 14.

In certain embodiments, the reflective layer 14 comprises the firstmaterial throughout its thickness, and the internal stress of the firstmaterial is modified during deposition by varying at least onedeposition parameter such that the internal stresses of the reflectivelayer 14 and the deformable layer 34 at the junction 145 aresubstantially equal. Examples of deposition parameters that may bevaried to modify the properties of the reflective layer 14 include, butare not limited to, temperature, pressure, power, deposition duration,the first material precursors, and the flowrate of the first materialprecursors.

As an example, in an embodiment in which the first material comprisesaluminum and the deformable layer 34 comprises nickel with an internalcrystal lattice stress of 350 MPa at the junction 145, the reflectivelayer 14 may comprise the first material with a first intrinsic crystallattice stress of 50 MPa at the second side 142 and a second intrinsiccrystal lattice stress of 300 MPa at the first side 141. At the junction145, the deformable layer 34 has an internal stress of 350 MPa and thereflective layer 14 has an internal stress of 300 MPa, so the internalstresses are substantially equal, in this embodiment with a differenceof 50 MPa.

In certain embodiments, the properties of the deformable layer 34 aremodified during deposition by varying at least one deposition parameter.As an example, in an embodiment in which the second material comprisesnickel and the reflective layer 14 comprises aluminum with an internalcrystal lattice stress of 50 MPa at the junction 145, the deformablelayer 34 may comprise the second material with a first intrinsic crystallattice stress of 350 MPa at a first side 143 distal from the reflectivelayer 14 and a second intrinsic crystal lattice stress of 100 MPa at asecond side 144 proximate to the reflective layer 14. At the junction145, the deformable layer 34 has an internal stress of 100 MPa and thereflective layer 14 has an internal stress of 50 MPa, so the internalstresses are substantially equal, in this embodiment with a differenceof 50 MPa.

In certain embodiments, the properties of both the reflective layer 14and the deformable layer 34 are modified during deposition. As anexample, the reflective layer 14 may comprise the first materialcomprising aluminum with a first intrinsic crystal lattice stress of 50MPa at the first side 141 and a second intrinsic crystal lattice stressof 200 MPa at the second side 142, and the deformable layer 34 maycomprise the second material comprising nickel with an intrinsic crystallattice stress of 350 MPa at the first side 143 and an intrinsic crystallattice stress of 200 MPa at the second side 144. At the junction 145,the deformable layer 34 has in internal stress of 200 MPa and thereflective layer 14 has an internal stress of 200 MPa, so the internalstresses are substantially equal, in this embodiment with a differenceof zero MPa.

In certain embodiments, the reflective layer 14 with a gradedcomposition comprises an alloy. In certain embodiments, the alloycomprises at least one element of the second material. For example, thealloy may comprise the first material comprising a reflective materialand the second material comprising a deformable material. The reflectivelayer 14 comprises substantially all reflective first material on a side142 of the reflective layer 14 facing away from the deformable layer 34and substantially all deformable second material on a side 141 proximateto the junction 145 of the reflective layer 14 and the deformable layer34. The deformable layer 34 comprises the deformable second material, sothe reflective layer 14 and the deformable layer 34 have substantiallyequal internal stresses at the junction 145.

In certain embodiments, the second side 142 of the reflective layer 14comprises substantially all aluminum and the first side 141 of thereflective layer 14 comprises substantially all nickel, with the ratioof aluminum to nickel decreasing within the reflective layer 14 from thesecond side 142 to the first side 141. The graded composition of thereflective layer 14 may comprise different ratios of the first materialto the second material by altering deposition parameters, for example,but not limited to, temperature, pressure, power, first and secondmaterial precursors, and the ratio of the flowrates of the first andsecond material precursors. For example, the precursors may comprisesubstantially all first material precursor, then an increasing amount ofsecond material precursor and a decreasing amount of first materialprecursor, then substantially all second material precursor. For anotherexample, the precursors may comprise substantially all first materialprecursor, then an increasing amount of second material precursor with aconstant amount of first material precursor, then substantially allsecond material precursor. In certain embodiments, the second side 142comprises aluminum with an intrinsic crystal lattice stress of 50 MPaand the first side 141 comprises nickel with an intrinsic crystallattice stress of 350 MPa.

When the reflective layer 14 is in a relaxed position, as depicted onthe left side of FIG. 14, the deformable layer 34 is in a relaxed state.When the reflective layer 14 is in an actuated position, as depicted onthe right side of FIG. 14, the deformable layer 34 is in a deformedstate. The second material can be optimized to restorably deform when aforce (e.g., in the direction of a distributed electrostatic force dueto an electric field as schematically indicated by arrows 146) attractsthe reflective layer 14 towards the substrate 20. When the force isremoved from the reflective layer 14, the deformable layer 34 returns tothe relaxed state and the reflective layer 14 returns to the relaxedposition (e.g., as illustrated on the left side of FIG. 14).

The curvature and/or tilt of the reflective layer 14 may also bedecreased by using combinations of the embodiments of FIGS. 13 and 14(e.g., a bilayer with one or both layers having a graded composition).FIG. 15 depicts a cross-sectional view of yet another embodiment of adevice in which the reflective layer 14 comprises a first material andin which the deformable layer 34 comprises a second material differentfrom the first material. The reflective layer 14 comprises a reflectivelayer 152, for example comprising aluminum with an intrinsic crystallattice stress of 50 MPa. The deformable layer 34 comprises, forexample, nickel with an intrinsic crystal lattice stress of 350 MPa. Thereflective layer 14 further comprises a graded composition having anintrinsic crystal lattice stress of 50 MPa at a side 153 distal to thejunction of the reflective layer 14 and the deformable layer 34 and anintrinsic crystal lattice stress of 300 MPa at a side 151 proximate tothe junction of the reflective layer 14 and the deformable layer 34. Thejunction of the reflective layer 14 and the deformable layer 34comprises materials having substantially similar internal stresses. Suchan embodiment advantageously allows any thickness of the reflectivelayer 152 in comparison to the thickness of the reflective layer 15.

When the reflective layer 14 is in a relaxed position, as depicted onthe left side of FIG. 15, the deformable layer 34 is in a relaxed state.When the reflective layer 14 is in an actuated position, as depicted onthe right side of FIG. 15, the deformable layer 34 is in a deformedstate. The second material can be optimized to restorably deform when aforce (e.g., in the direction of a distributed electrostatic force dueto an electric field as schematically indicated by arrows 156) attractsthe reflective layer 14 towards the substrate 20. When the force isremoved from the reflective layer 14, the deformable layer 34 returns tothe relaxed state and the reflective layer 14 returns to the relaxedposition.

Other MEMS devices, for example switches, may also benefit from theoptimization of the structural design and materials used to form thedevice, and may result in different materials being used for adeformable layer and a functional layer connected to the deformablelayer that is preferably flat. Similar to the reflective layer ininterferometric modulators, the use of different materials that havedifferent properties can cause curvature and/or tilt in the functionallayer.

FIGS. 16A through 16D depict an example switch, which may also be calledan “ohmic switch,” a “series switch,” a “MEMS relay,” or other suitablenames, with different materials for a contact layer 162 and a deformablelayer 34 to which the contact layer 162 is connected and/or differentmaterials for an actuation electrode 168 and the deformable layer 34 towhich the actuation electrode 168 is connected. FIGS. 16A and 16Cillustrate cross-sectional side views of a switch in a relaxed positionand an actuated position, respectively, in which the contact layer 162and the actuation electrode 168 comprise a first material and thedeformable layer 34 comprises a second material. FIGS. 16B and 16Dillustrate cross-sectional front views of the switches of FIGS. 16A and16C, respectively.

Benefits may derive from using different materials for the contact layer162 and/or the actuation electrode 168 and for the deformable layer 34,for example to decouple the electrical properties of the contact layer162 and/or the actuation electrode 168 from the mechanical properties ofthe deformable layer 34. For example, the structural design andmaterials used for the contact layer 162 and/or the actuation electrode168 can be optimized with respect to the electrical properties, and thestructural design and materials used for the deformable layer 34 can beoptimized with respect to desired mechanical properties. In certainembodiments, the contact layer 162 and/or the actuation electrode 168comprises a conductive material (e.g., aluminum, copper, gold) and thedeformable layer 34 comprises an insulating material (e.g., SiO₂,SiN_(x)).

However, curvature and/or tilt of the contact layer 162 and/or theactuation electrode 168 (as depicted in FIGS. 16A through 16D) resultingfrom stress gradients may affect the electromechanical response of theswitch. For example, different distances between the actuation electrode168 and the electrode 166 can affect the response time and actuationvoltage of the switch. The actuation voltage of the switch may be setaccording to the distance between the actuation electrode 168 and theelectrode 166, so a change in the distance between the actuationelectrode 168 and the electrode 166 may cause the device to functionimproperly or to fail. Moreover, the surface of the contact layer 162facing the substrate 20 is preferably substantially parallel to thesubstrate 20. However, curvature and/or tilt of the contact layer 162may cause the surface of the contact layer 162 facing the substrate 20to become non-parallel to the substrate 20. The contacting portions ofthe contact layer 162 may not make full contact with the leads 163, 164(as depicted in FIGS. 16C and 16D), thereby impeding the conductance ofan electrical signal.

FIGS. 17A and 17C illustrate cross-sectional side views of an embodimentof a switch in a relaxed position and an actuated position,respectively, in which the contact layer 162 and the actuation electrode168 comprise a first material and the deformable material 34 comprises asecond material different from the first material. FIGS. 17B and 17Dillustrate cross-sectional front views of the switches of FIGS. 17A and17C, respectively. In accordance with an embodiment described above withrespect to FIG. 9F, a connecting elements 169 is mechanically coupled(e.g., fused) to the deformable layer 34 and the contact layer 162 andthe deformable layer 34 and a connecting element 171 is mechanicallycoupled (e.g., fused) to the actuation electrode 168 and the deformablelayer 34. The connecting elements 169, 171 form interfaces 165, 167between the first material and the second material. The connectingelements 169, 171 may be the same or the connecting element 169 may bedifferent from the connecting element 171. The interfaces 165, 167between the first material and the second material are spaced from thecontact layer 162 and the actuation electrode 168, respectively, therebycausing the distances between the contact layer 162 and the leads 163,164 to be substantially the same and the surface of the contact layer162 facing the substrate 20 to be substantially parallel to thesubstrate 20 and causing the distance between the actuation electrode168 and the electrode 166 to be substantially the same and the surfaceof the actuation electrode facing the substrate 20 to be substantiallyparallel to the substrate 20. It will be understood that the switch mayalso be in accordance with the embodiments depicted in FIGS. 10 through12, combinations thereof, and the like. For example, the connectingelement 171 may be fused to the actuation electrode 168 while theconnecting element 169 is adhered to the contact layer 162.

FIGS. 18A and 18C illustrate cross-sectional side views of anotherembodiment of a switch in a relaxed position and an actuated position,respectively, in which the contact layer 162 and the actuation electrode168 comprise a first material and the deformable material 34 comprises asecond material different from the first material. FIGS. 18B and 18Dillustrate cross-sectional front views of the switches of FIGS. 18A and18C, respectively. In accordance with an embodiment described above withrespect to FIG. 13, the deformable layer 34 is mechanically coupled tothe contact layer 162 and the actuation electrode 168 at junctions 175,177, respectively. The contact layer 162 and the actuation electrode 168have substantially equal internal stresses with the deformable layer 34at the junctions 175, 177, thereby causing the distances between thecontact layer 162 and the leads 163, 164 to be substantially the sameand the surface of the contact layer 162 facing the substrate 20 to besubstantially parallel to the substrate 20 and causing the distancebetween the actuation electrode 168 and the electrode 166 to besubstantially the same and the surface of the actuation electrode facingthe substrate 20 to be substantially parallel to the substrate 20. Itwill be understood that the switch may also be in accordance with theembodiments depicted in FIGS. 14 and 15, combinations thereof, and thelike. For example, the actuation electrode 168 may comprise a bilayerwhile the contact layer 162 has a graded composition that varies from afirst side of the contact layer 162 proximate to the deformable layer 34to a second side of the contact layer 162 distal from the deformablelayer 34.

Various specific embodiments have been described above. Although theinvention has been described with reference to these specificembodiments, the descriptions are intended to be illustrative of theinvention and are not intended to be limiting. Various modifications andapplications may occur to those skilled in the art without departingfrom the true scope of the invention as defined in the appended claims.

1. A microelectromechanical (MEMS) device comprising: a functional layer comprising a first material; and a deformable layer comprising a second material different from the first material, the deformable layer mechanically coupled to the functional layer at a junction, wherein the functional layer and the deformable layer have substantially equal internal stresses at the junction.
 2. The MEMS device of claim 1, wherein an internal stress of the functional layer at the junction differs from an internal stress of the deformable layer at the junction by less than about 150 MPa.
 3. The MEMS device of claim 1, wherein an internal stress of the functional layer at the junction differs from an internal stress of the deformable layer at the junction by less than about 60 MPa.
 4. The MEMS device of claim 1, wherein an internal stress of the functional layer at the junction differs from an internal stress of the deformable layer at the junction by less than about 10 MPa.
 5. The MEMS device of claim 1, wherein the functional layer has a thickness greater than about 1 micron.
 6. The MEMS device of claim 1, wherein the functional layer and the deformable layer have substantially equal coefficients of thermal expansion at the junction.
 7. The MEMS device of claim 1, wherein the first material comprises a conductive material.
 8. The MEMS device of claim 1, wherein the first material comprises aluminum.
 9. The MEMS device of claim 1, wherein the second material comprises nickel.
 10. The MEMS device of claim 1, wherein the functional layer comprises a contact layer comprising the first material on a side of the functional layer facing away from the deformable layer.
 11. The MEMS device of claim 1, wherein the functional layer comprises a reflective layer comprising the first material on a side of the functional layer facing away from the deformable layer.
 12. The MEMS device of claim 11, wherein the functional layer comprises a bilayer comprising the reflective layer and a layer comprising the second material.
 13. The MEMS device of claim 12, wherein the reflective layer is less than about 20% as thick as the layer comprising the second material.
 14. The MEMS device of claim 12, wherein the reflective layer is less than about 10% as thick as the layer comprising the second material.
 15. The MEMS device of claim 12, wherein the reflective layer is less than about 3% as thick as the layer comprising the second material.
 16. The MEMS device of claim 12, wherein the first material comprises aluminum and wherein the second material comprises nickel.
 17. The MEMS device of claim 1, wherein the functional layer has a graded composition that varies from a first side of the functional layer proximate to the deformable layer to a second side of the functional layer distal from the deformable layer.
 18. The MEMS device of claim 17, wherein the functional layer is reflective at the second side of the functional layer.
 19. The MEMS device of claim 17, wherein the functional layer comprises an alloy.
 20. The MEMS device of claim 19, wherein the alloy comprises at least one element of the second material.
 21. The MEMS device of claim 19, wherein the alloy comprises nickel and aluminum.
 22. The MEMS device of claim 1, further comprising: a display; a processor configured to communicate with the display, the processor configured to process image data; and a memory device configured to communicate with the processor.
 23. The MEMS device of claim 22, further comprising a driver circuit configured to send at least one signal to the display.
 24. The MEMS device of claim 23, further comprising a controller configured to send at least a portion of the image data to the driver circuit.
 25. The MEMS device of claim 22, further comprising an image source module configured to send the image data to the processor.
 26. The MEMS device of claim 25, wherein the image source module comprises at least one of a receiver, transceiver, and transmitter.
 27. The MEMS device of claim 22, further comprising an input device configured to receive input data and to communicate the input data to the processor.
 28. A microelectromechanical (MEMS) device comprising: means for controlling a signal, the controlling means comprising a first material; and means for supporting the controlling means, the supporting means comprising a second material different from the first material, the supporting means mechanically coupled to the controlling means at a junction, wherein the controlling means and the supporting means have substantially equal internal stresses at the junction.
 29. The MEMS device of claim 28, wherein the supporting means comprises a mechanical layer.
 30. The MEMS device of claim 28, wherein the controlling means comprises a reflective layer that controllably reflects an electromagnetic signal.
 31. The MEMS device of claim 28, wherein the controlling means comprises a conductive layer that controllably conducts an electrical signal.
 32. A method of manufacturing a microelectromechanical (MEMS) device on a substrate, the method comprising: forming a functional layer comprising a first material; forming a sacrificial layer comprising sacrificial material over the functional layer; forming a hole in the sacrificial layer to expose a portion of the functional layer; and forming a deformable layer comprising a second material over the sacrificial layer, the second material different from the first material, the deformable layer fusing to the functional layer at a junction in the exposed portion of the functional layer, wherein the functional layer and the deformable layer have substantially equal internal stresses at the junction.
 33. The method of claim 32, further comprising removing the sacrificial layer.
 34. The method of claim 32, wherein forming the functional layer comprises: depositing a reflective material comprising the first material; and depositing the second material over the reflective material.
 35. The method of claim 32, wherein forming the functional layer comprises: depositing a conductive material comprising the first material; and depositing the second material over the reflective material.
 36. The method of claim 32, wherein forming the functional layer comprises depositing the first material while modifying a deposition parameter, the functional layer having a graded composition that varies from a first side of the functional layer proximate to the deformable layer to a second side of the functional layer distal from the deformable layer.
 37. The method of claim 36, wherein modifying includes varying at least one of temperature, pressure, and power.
 38. The method of claim 32, wherein the functional layer comprises an alloy.
 39. The method of claim 38, wherein the alloy comprises at least one element of the second material.
 40. The method of claim 38, wherein the alloy comprises nickel and aluminum.
 41. The method of claim 38, wherein modifying includes varying a ratio of a flowrate of a first material precursor to a flowrate of a second material precursor.
 42. The method of claim 32, wherein forming the hole in the sacrificial layer comprises: forming a patterned layer over the sacrificial layer; etching the sacrificial material uncovered by the patterned layer; and removing the patterned layer.
 43. The method of claim 42, wherein etching the sacrificial material comprises selectively etching the sacrificial material and negligibly etching the functional layer.
 44. A microelectromechanical (MEMS) device fabricated by the method of claim
 32. 